The present invention relates to electric arc furnaces for melting scrap metal, and more particularly to oxy-fuel burner-assisted furnaces. In accordance with the invention, methods of modifying and operating the furnaces are disclosed wherein the burner combustion gases form a circular swirling flow path within the furnace and provide an endless or continuous circular melt trench within the scrap metal.
Electric arc furnaces are used in iron and steel foundries, steel mills, and non-ferrous melt applications. Direct arc furnaces wherein current is passed directly from a power source through the metal are used for their ability to bring the scrap metal or burden material to be melted quickly to pour temperature. Typically, the furnace has a cylindrical side wall which surrounds a central electrode arrangement. The furnace roof and electrode arrangement are removable for charging the furnace through its open top. The furnace is sized to receive its total charge as a plurality of fractional charges deposited in the furnace by an overhead scrap bucket. The fractional charges are sequentially melted. Examples of such furnaces are disclosed in U.S. Pat. Nos. 3,472,649; 4,455,660; and 4,564,950. Herein, a Heroult-type furnace having three centrally located electrodes arranged in a triangular or delta pattern is described in connection with steel-making for purposes of illustration.
Electric arc furnaces are characterized by "cold spots" and uneven melt pattern which are related to the structural features of the furnace and scrap bucket, as well as the scrap material density and/or gross configuration. For example, such cold spots are related to the trefoil-shaped heating pattern of the delta electrode arrangement, as well as other furnace structures such as the slag door and fourth-hole evacuation port locations. Since such structural design features vary, the cold spot locations are not uniform from furnace to furnace. Variations in the positioning of scrap metal charges are also encountered. Furnace diameter, scrap bucket diameter relative to the furnace diameter, and the degree of pivoting of the furnace roof to an open position vary, and tend to result in unique scrap loading patterns for each furnace. Similarly, the composition of the scrap material itself also tends to cause variations in the required heat distribution.
The existence of cold spots and uneven melt patterns may result in energy-inefficient melting of the scrap by the chance distribution of the scrap within the furnace during charging including the possible deposit of excess scrap adjacent a cold spot. Further, the prolonged electrode heat cycle required to melt scrap disposed in a cold spot may in turn impose undue wear damage on exposed side wall portions of the furnace in scrapfree locations, due to the direct exposure to the electrodes. The heat lost from the high temperature gradient across the exposed portions of the side wall adds to the total energy required for the heat.
Oxy-fuel burners which burn a mixture of oxygen and natural gas have been used in electric arc furnaces to assist melting and to heat cold spots located on the basis of furnace and/or electrode positions. For example, in delta electrode patterns the cold spots are located intermediate the adjacent electrodes. In order to heat such cold spots, the prior art discloses the mounting of oxy-fuel burners radially through the furnace side wall to fire directly into the cold spots between adjacent electrodes. Such arrangements have not been found satisfactory in most instances, since they tend to simply divide the cold spot into two smaller and spaced cold spots. The mounting of burners through the furnace roof to fire directly down into the cold spots has also failed to provide satisfactory results.
The use of circumferentially directed oxy-fuel burners mounted through a ring disposed below the furnace roof for impingement of the scrap metal with burner flame jets is disclosed in U.S. Pat. No. 3,459,867. Applicants' own prior art efforts including mounting oxy-fuel burners through the furnace roof in a circumferential pattern similar to that in such patent. In three-electrode furnaces, the roof openings were radially aligned with the electrodes where structurally possible and arranged to fire downwardly at about a 45-degree angle across the cold spot. A cold spot remote from the slag door was assumed to lie on a radial line bisecting the angle between adjacent electrodes and located midway between the electrode outer perimeter and the furnace side wall. The cold spot adjacent a furnace slag door was determined by observation of unmelted scrap to be angularly offset from the radial line intermediate the adjacent electrodes. The operation of these furnaces by applicants did not necessarily result in the formation of a melt trench as provided in the present invention. Applicants did not attempt to provide such melt trench until the discovery of the invention herein.
It has now been discovered that the judicious alignment and heat energy regulation of oxy-fuel burners in an electric arc furnace provide a circular swirling flow of combustion gases which establish an endless melt trench which extends in a circular configuration within the scrap metal about the center of the furnace. Thus, it has been learned that furnace cold spots and/or irregular melt patterns are repeatable or reproducible characteristics of particular furnace installations which can be substantially eliminated and replaced by melt trench operation through individual burner adjustment. The burner firing alignments and energy inputs are adjusted to establish the melt trench based upon observing or monitoring the furnace operation over a period of time to determine furnace cold spots and melt patterns.
In accordance with the invention, different types of burner adjustments are made in particular sequences to assure the provision of the melt trench. The melt trench tends to minimize the inefficient use of electrodes to melt scrap at the perimeter of the furnace and also avoids the excessive wear and damage to the side walls associated with unnecessary direct exposure to electrode radiation.
The melt trench may be used further to optimize the furnace operation, including the provision of energy efficiency improvements and regulation of overall desired characteristics, such as steel demand rates in a particular shop and the relative costs of energy. To that end, the furnace operation is further observed and burner as well as certain electrode operating adjustments are made in order to provide an even or uniform melt characterized by substantially simultaneous completion of scrap melting by the electrode and by the burners. Preferably, the electrodes complete melting of the scrap generally located at the center of the furnace at about the same time the burners complete the melting of radially outward scrap located at the side wall of the furnace.
In accordance with the invention, an electric arc furnace is modified for gas burner assistance by the use of oxy-fuel burners of specific heating and combustion gas velocity characteristics to provide a melt trench. The burners are mounted through the roof to flow combustion gases across associated cold spots, establish circumferential flow of the combustion gases within the furnace, and affect as large an amount of scrap metal with as much energy as possible early in the melt cycle.
A plurality of burners are typically used, one being associated with each cold spot. As permitted by the furnace structure, burners are mounted through the furnace roof at angularly spaced locations upstream from associated cold spots. The burners are swivel-mounted to allow movement through a cone angle of less than about 5 degrees. Such a mounting has been found sufficient to accommodate deviations in mounting point due to furnace structure interference and to enable burner alignment adjustments for effecting the melt trench, even melt operation, and customary optimization variations.
The provision of the melt trench includes initial adjustment of the gross burner energy output, lateral burner firing direction or angle alignment with respect to the center of the furnace, and elevational burner firing direction or angle alignment with respect to the vertical. These adjustments are made in accordance with visual furnace inspections and sensed operating conditions to provide an endless melt trench within the scrap metal and to eliminate side wall wear spots as well as unmelted scrap locations.
The melt trench is a toroidal or donut-shaped melt zone within the scrap metal adjacent the upper surface thereof, and generally extending around the electrodes halfway between them and the furnace side wall. Depending upon the scrap height within the furnace, the trench may include initial tunnel portions extending through the scrap metal which subsequently collapse to provide an open trench corresponding in shape with a toroid sectioned at its mid-plane. It is believed that the melt trench provides a site of rapid and efficient melting, since the swirling combustion gases traveling along the trench more effectively contact the scrap metal at the trench wall over a prolonged period of time. Red steel is observed along the trench bottom as the melting progresses and molten metal continuously drains downwardly from the trench to the furnace bottom. As the trench melts downwardly through the height of the scrap metal pile, scrap metal falls into the trench from the sides thereof and scrap is cleared from the furnace side wall.
As indicated above, the efficiency of the furnace operation and/or special optimized operating characteristics may be achieved by the use of the melt trench. For example, electrical energy consumption has been reduced by as much as 18%. Also, production rates have been increased by reduction of the tap-to-tap time. Efficiency may be increased by even melt, which is effected by delaying the burner firing until sufficient electrode penetration and melting have occurred. In particular contrast with less effective prior art techniques, the burners are ignited prior to the electrode firing in the above-noted U.S. Pat. No. 3,459,867. Once an appropriate burner firing delay has been determined, the energy input of individual burners is adjusted to substantially eliminate disjuncted accumulations of unmelted scrap metal within the furnace.